1. Field of the Invention
This invention relates to a serial input/output system applicable to small computers and terminals and more particularly a priority access mechanism for such an input/output system.
2. Description of the Prior Art
A number of different input/output interfaces have been developed for a variety of both high and low speed data processing systems. When the system employs high speed peripheral devices such as magnetic discs and high speed tape drives, data transfer across the interface is ordinarily parallel by byte or by word. The relatively high cost of the large number of parallel conductors required for such an interface is justified by the high data transfer rate that is achieved. Such high speed interfaces usually include extra conductors for device addresses and to handle access or interrupt requests according to the priority of the device requesting access or interrupt. Attached to such a high speed interface may be a peripheral control unit which in turn communicates with slow speed I/O devices such as display units, printers and keyboards. The data communication between the controller and such slow speed devices can be accomplished serially by bit. However, the handling of access or interrupt requests normally requires additional request and device address conductors. Small data processing systems such as employed in small businesses and locations remote from the central processor of larger organizations also sometimes are provided with a slow speed interface to communicate with display devices, keyboards and the like.
Such terminal controller units often service a large number of different types of small I/O devices which require a plurality of different communication lines between the individual devices and the controller. This increases the cost of the controller unit. Furthermore, the controllers often require modification to incorporate anticipated new devices. The general result is a lack of standardization of cables and connectors and the communication discipline.
In order to service all the respective devices, the controller usually polls the unit in a sequence starting with the highest priority device. This type of servicing arrangement provides for rather low data throughput as the polling sequence is fixed such that if a low priority device is requesting service more often than a high priority device, the controller must still repeat the polling sequence.
It is then an object of the present invention to provide an improved input/output system which is low in cost but nevertheless can handle a large number of low-speed input/output devices.
Another object of the present invention is to provide an improved input/output system having an access request mechanism which does not add to the cost of the input/output communication bus.
It is a further object of the invention to provide an inexpensive input/output interface having an access request mechanism for a wide variety of input/output devices.